Honestly, a lot of being "quick on your feet" means having a sharp wit, but just as importantly, it requires practice. GETTING OUT THERE and building up comfort and experience is the key first step. However, it is also helpful to think up snappy one-liners every now and then or picking them up on sites like this and keeping them in the back of your head so you can bust them out when the need arises. For instance, if you're short, have a few good contextual comebacks ready for that. The reality is, a lot of the snappy comebacks you quoted probably weren't just made up with right on the spot and out of the blue - they probably heard them before or had time to put them together from their own experience. Over time, it gets easier to adapt these as the situation calls for it, but you'll also find it's a lot easier to act naturally and not rely on "canned material".
Think of it like boxing: you never know what punches the other guy is going to throw at you or how he's going to be moving away from your punches, so there's a ton of improvisation and quick thinking involved. However, boxers spend YEARS in the gym practicing techniques over and over, so when fight night comes, they just adapt that knowledge to the circumstances. Eventually, rehearsing this stuff just comes naturally.
Haha, now I feel better: I've finally managed to throw in boxing analogy to another facet of life - one of my quirks is to make some weird-ass comparisons so I can talk about my favourite sport
